1. Field of the Invention
The invention relates to heat storage material such as those that are used in solid heating applications for heating interior spaces of buildings and the like. The invention more specifically relates to encapsulated phase change material for use as such heat storage material and to the processes of preparing and using such encapsulated phase change material.
2. Background
Low temperature thermal energy storage is essential for matching the supply of solar energy to the demands for space and water heating. Combined, these demands account for more than two-thirds of the total residential energy usage in the United States.
A widely-used method for the storage of heat in solar energy applications involves the use of water as a heat storage medium, wherein the water is heated by the sun and is later cooled to release the stored heat as it is needed. Water has been widely used for this purpose because it is readily available and has low corrosivity. However, in order to store practical amounts of heat, a large volume of water is necessary, requiring that large areas be dedicated to water storage.
Another method involves the use of rocks as a heat storage medium, generally in conjunction with a pumped fluid, such as air, to carry the heat to and from the storage area where the rocks are contained, the solar energy collectors and rooms to be heated. As with heat storage with water, heat storage with rocks requires a large volume of rocks to be practical.
In a process where the medium does not go through a phase-change during the heating and cooling cycle, such as those using water or rocks, the heat is stored as an increase in enthalpy of the storage medium as it is heated. Heat is released as the storage medium cools and the enthalpy of the medium is lowered.
This can be mathematically illustrated by equation (1): ##EQU1## wherein Q is the amount of heat released or absorbed as the medium is cooled or heated between temperatures T.sub.1, and T.sub.2, i.e., the amount of the heat stored in the medium during the heating-cooling cycle, M is the mass of the medium, T.sub.2 is the highest temperature of the heating-cooking cycle, T.sub.1 is the lowest temperature of the heating-cooling cycle, C.sub.pm the constant pressure heat capacity of the heat storage medium and dT the differential temperature. Generally in the temperature ranges involved in solar heat storage the heat capacity is approximately constant, therefore, the simpler equation employed is: EQU Q=M C.sub.pm (T.sub.2 -T.sub.1) (2)
It can be seen from the above equations that there must be a change in temperature for heat to be absorbed or released and the amount of heat stored is proportional to the heat capacity. Thus, the amount of heat that can be stored is dependent on the temperature range of the heating and cooling cycle and also upon the magnitude of the heat capacity.
Because of the inherent magnitudes of the heat capacities of substances currently or previously used as single phase heat storage media and the temperature ranges employed in heat storage methods, generally the only way to store practical amounts of heat in a single-phase process is to have a large mass of the heat storage medium.
Since such a mass is undesirable in the typical case, it has been proposed to use a medium that changes phase during the heating and cooling cycle. Generally suggested are media that would melt upon being heated and solidify upon being cooled. In such a process the heat would be stored in the form of the heat of fusion of the medium. During the change in phase, heat would be released or absorbed without a change in temperature of the medium as it is freezing or melting. The heat storage process using such a phase-change medium can be represented mathematically as: ##EQU2## wherein C.sub.ps and C.sub.pl are the heat capacities per unit mass for the solid and liquid respectively, T.sub.m is the temperature of the melting point of the medium, h.sub.f is the heat of fusion, and Q, M, dt, T.sub.1, and T.sub.2 are defined as above. Assuming, the heat capacities are approximately constant, then: EQU Q=M[(T.sub.2 -T.sub.m)C.sub.pl +h.sub.f +(T.sub.m -T.sub.1)C.sub.ps ](4)
If the heat of fusion is high and the temperature range is narrow, then the heat capacity terms in Equation (4) are not significant and it can be further simplified to: EQU Q=M h.sub.f ( 5)
From Equation (5) it can be seen that if the heat of fusion is high, the mass of the medium can be proportionately reduced. It can also be seen that the release or absorption of heat does not require a change in temperature of medium while it is freezing or melting. Thus, a substance may be useful as a phase-change heat storage medium if it melts and solidifies within the temperature range encountered in solar heat storage methods. This would be from about room temperature (20.degree. to 25.degree. C.) to temperatures obtainable from solar heat collection devices, i.e., about 50.degree. C.
A suitable phase-change heat storage medium or phase change material (PCM) would also have a high heat of fusion in order to reduce the mass or the volume required to store the necessary amount of heat. Also, if the heat of fusion of the medium is high, the temperature range of the heating and cooling cycle need only be in a narrow range encompassing the melting temperature to provide for the storage of relatively large amounts of heat.
Certain paraffins have been suggested as phase-change media, as for example in Bruce Anderson and Michael Riondan, "The Solar Home Book", Brick House Publishing Co., Harrisville, Hampshire, at p. 191. The paraffins suggested are certain paraffins commercially available for use in solar heat applications. These paraffins are cheaper than eutectic salts and have a relatively high heat of fusion of around 36 calories per gram. However, they have their own unique problems. Such paraffins degrade plastics and corrode some metals such as copper. When exposed to air, hot paraffins slowly oxidize or degrade to form more corrosive oxidation products, such as organic acids. Another difficulty, as reported by Anderson et al., lies in the fact that paraffin expands by about 20 percent when it melts, thereby causing problems of containment. When it solidifies and releases stored heat, the paraffin shrinks and pulls away from the container wall, drastically slowing the rate of the transfer of heat out of the container.
Certain eutectic salts have also been proposed as heat storage media because of their high heats of fusion and their melting points within the appropriate range. These salts are particularly desirable because of their high heats of fusion, thus they are usable in small volume applications. Among the salts suggested are sodium sulfate decahydrate (melting point 32.degree. C., heat of fusion 51 cal/g), calcium chloride hexahydrate (melting point 30.degree. C., heat of fusion 40.7 cal/g), sodium carbonate decahydrate (melting point 32.5 to 34.5.degree. C.), calcium nitrate tetrahydrate (melting point 39.7.degree. to 2.7.degree. C.) and sodium thiosulfate pentahydrate (melting point 40.degree. to 45.degree. C.).
However, eutectic salts are extremely corrosive when compared to water or organic phase-change media, such as the paraffins discussed above. Also, the proposed salts are hydrates It is well known that hydrated salts, when exposed to varying humidity and temperature conditions, will change their composition or degree of hydration. In eutectic salts, this results in a solid phase that does not melt at an appropriately low temperature. Thus, when used as phase-change heat storage media, eutectic salts cannot be stored in conventional storage tanks but must be packaged in special non-corrosive containers that are sealed to maintain the composition of the salts. R. H. Montgomery and J. K. Budnik, "The Solar Decision Book," Dow Corning Corporation, Midland, Mich., at page 14-5, referring to eutectic salts and paraffins, states the following:
"Phase-changing materials meet the requirements for small-volume heat storage. However, they are not commercially feasible at this time. The costs of properly packaging these materials in some type of container that would work for solar storage are currently prohibitive. There are performance problems as well. The materials work only within a narrow temperature range, and they require frequent replacement." PA1 "These shortcomings of phase-changing materials plus the lack of other suitable storage concepts - leaves water and rocks as the best solar storage choices at this time. Water and rocks should continue as the most popular storage materials for at least the next five years."
U.S. Pat. No. 4,360,442 discloses a process for the storage of heat involving heat transfer to and from a heat storage medium wherein the heat storage medium changes phase as it absorbs or releases heat. Ethylene carbonate-containing hexamethylcyclotrisiloxane is used as the heat storage medium.
Phase change materials, such as salt hydrates, paraffin, napthalene, and crystalline polymers like high density polyethylene and CARBOWAX.RTM. polyethylene glycol 8000, (Union Carbide Corporation, Danbury, Conn., U.S.A.) accomplish low temperature thermal energy storage and offer potential size, weight and overall cost advantages when measured against sensible heat storage schemes. If they could be stabilized in small particle sizes, e.g., 5 to 10 mm in diameter or smaller, heat transfer to and from phase change materials (PCMs) is dramatically improved owing to the increased surface to volume ratio. This size range makes phase change materials amenable to use in active heat exchange systems, such as, packed bed exchangers, in which a heat transfer fluid circulates through the particle bed. It also makes them available for passive designs where the phase change material is incorporated into building materials, such as, floors, walls and ceilings.
There are at least two ways of reducing phase change materials to a useful particle size range and maintaining their particulate character through repeated freeze/thaw cycling. One is to render the particles "form stable" by chemically crosslinking the phase change material molecules, an approach practical only with organic, high molecular weight phase change materials. The object is to retard the flowability of the phase change material in its molten state without materially reducing its heat of fusion. The art has mildly crosslinked high density polyethylene and produced nonspherical particles roughly 6 mm across. See: Salyer et al., I.O., "Form Stable, Crystalline Polymer Pellets For Thermal Energy Storage," Proc. Intersoc. Energy Conversion Eng. Conf., 13(2), 948-962 (1978); and Whitaker, R. B., et al., "Energy Storage For Solar Air Conditioning Applications Utilizing A Form-Stable, High Density Polyethylene Pellet Bed," NTIS CONF 790328-1 (1978).
A second way to impart "form stability" is through encapsulation of the phase change material. See U.S. Pat. No. 4,219,072. Mebalick, E. M., and A. T. Tweedie, "Two Component Thermal Storage Material Study: Phase II," NTIS COO/2845-78/2 (May 1979) encapsulated paraffin in the 100 to 1000 micron particle size range. Employing an interfacial polymerization technique, they formed polyamide shells around wax cores. Their goal was to demonstrate the utility of the encapsulated phase change material in a fluidized bed heat exchanger using water as the heat transfer fluid. Plugging of the heat exchanger, due primarily to fine particles (less than 100 micron diameter), prevented their completing the original experimental program. Repeated freeze/thaw cycling of quiescent particles showed no signs of mechanical failure of the polyamide shells.
A published example in which the thermal properties of the encapsulated phase change materials were measured in subsequent freeze/thaw cycling was reported in Frost, C. E., and T. L. Vigo, "Salts Raise Specific Heats Of Hollow Fibers", C&E News, page 67, (Sept. 7, 1981). Researchers at the USDA Textiles and Clothing Laboratory introduced various salt hydrates into preformed hollow fibers made of polypropylene or rayon and then determined the heat capacity of the fibers in a differential scanning calorimeter (DSC). Glauber's salt, Na.sub.2 SO.sub.4.10H.sub.2 O, with borax as the nucleation agent, apparently lost some of its heat of fusion following imbibition into polypropylene fibers. They stated that CaCl.sub.2.6H.sub.2 O/SrCl.sub.2.6H.sub.2 O contained in polypropylene fibers, the same salt hydrate combination in rayon hollow fibers, and Na.sub.2 SO.sub.4.10H.sub.2 O/borax in rayon fibers "exhibited desirable thermal characteristics through at least five heating and cooling cycles."
Pennwalt took another look at phase change material encapsulation in Clen, J., et al., "Pelletilization And Roll Encapsulation Of Phase Change Materials", Proc. 6th Annual Thermal And Chemical Contractors' Review Meeting, (Pub. 1982), pages 177 to 184. This time, researchers studied the pelletilization and then roll encapsulation of wax and several salt hydrates. Water-borne latexes were the principal coating material. The mechanical integrity of the polymer shells was maintained after the thermal cycling of wax, Glauber's Salt, and Na.sub.2 S.sub.2 O.sub.3.5H.sub.2 O particles in quiescent water/ethylene glycol fluids and Na.sub.2 HPO.sub.4.12H.sub.2 O particles in a calcium chloride solution. No testing of the heats of fusion was cited, either following encapsulation or thermal cycling.
Interfacial polymerization has been employed to form films over other solid substrates as well. Whitfield, Miller and Wasley published several articles and received several patents for a technique to impart shrink resistance to wool. Their patents report other fibrous materials to which the films can be applied. Whitfield, R. E., et al., "Stabilization Of Wool Fabric By Interfacial Polymerization", Textile Res. J., Vol. 31, p. 74, (1961), reported the coating of wool fibers by polyamides, in particular, poly(hexamethylene sebacimide). Their best results followed immersing the fiber first in an 8 percent aqueous solution of hexamethylene diamine for 15 seconds and then in a 2 percent solution of sebacoyl chloride in carbon tetrachloride for the same time. Whitfield, R. E., et al., "Wool Fabric Stabilization By Interfacial Polymerization", Textile Res. J., Vol. 31, pp. 704-712, (1961). Subsequent work demonstrated that a significant feature of their discovery was the chemical bonding of the polymer to the fiber. Although not all of the polymer was grafted, the grafted fraction alone was responsible for the enhanced shrink resistance--see Whitfield, R. E., et al., "Interfacial Polycondensation, I. The Formation Of Surface Graft Polymers On Wool," J. Appl. Poly. Sci., 8, 1607-1617 (1964). Whitfield, R. E, et al., "Wool Fabric Stabilization By Interfacial Polymerization. Part III. Polyurethanes," Textile Res. J., 32, 743-750 (1962), involves work in the same area using polyurethanes. Whitfield, R. E., et al., "Wool Stabilization By Interfacial Polymerization. Part IV. Polyureas, Polyesters, Polycarbonates, And Further Studies On Polyamides," Textile Res. J., 33, 440-444 (1963), involves further work in the same area using polyureas, polyesters and polycarbonates. Whitfield, R. E., et al., "Wool Fabric Stabilization By Interfacial Polymerization. Part V. Copolymers," Textile Res. J. 33, 752-754 (1963), also involves work in the same area using copolymers. U.S. Pat. Nos. 3,078,138, 3,079,216, 3,079,217, 3,084,018, 3,084,019 and 3,093,441 resulted from such work.
U.S. Pat. No. 3,078,138 claims, for example, a process for treating a fibrous material which comprises serially depositing on the fibrous material in superposed phases in interfacial relationship a pair of complementary, direct-acting organic, polyamide-forming intermediates. At least one of the phases is liquid. The intermediates directly react under the conditions to form a polyamide in situ on the material. Patent '138 asserts that the polymers formed on the wool fibers are not merely physical coatings and that they are chemically bonded to the wool, that is, the added polymer is grafted onto the wool. The other patents in that series are similar in disclosure and claims.
U.S. Pat. No. 3,143,405 deals with encasing glass fibers, moving at speeds in excess of 4000 feet per minute, in polyamides. The fibers are first immersed in a 10 percent solution of hexamethylenediamine in water and then in a 10 percent solution of adipyl chloride in carbon tetrachloride. The polyamide coating, or "size," prevents mutual abrasion of the glass strands and also promotes lubrication to aid in spinning and weaving operations.